Start with the most likely cause: a dried-out trap
Every basin, bath, shower and toilet has a U-bend or P-trap that holds a small amount of water. That water seal is what stops drain smells rising back up into the room. If a fitting has not been used for a while, such as a shower in a spare bathroom or an overflow gully outside, the water in the trap can evaporate and the smell of the drain below comes straight through.
This is the first thing to check because it is the most common cause and the easiest to rule out. Run water down every plughole and flush the toilet, leave it for ten minutes, then see if the smell has gone. If it has, you have found the cause and the fix is simply to run water through infrequently used fittings every week or two.
Shower waste traps often have a shallow seal
Shower traps, particularly older or shallow ones fitted under a low-profile tray, often hold much less water than a basin or bath trap and can dry out faster, especially after a holiday or a period without use. A smell that is noticeably worse near the shower, and improves after running the shower for a minute, points strongly to this.
If the smell returns quickly after running water, the trap may be too shallow for the situation or may have a worn seal, and it is worth having the trap checked rather than just topping it up repeatedly.
Biofilm build-up inside the waste pipe
Over time, soap residue, skin cells, hair and shower gel build up as a slimy biofilm on the inside of waste pipes, particularly shower and basin wastes. This build-up can smell musty or like drains even when the trap itself is holding water properly.
This is safe to tackle yourself. Remove any visible hair from the plug or waste cover, then flush the waste with a proprietary pipe or drain cleaner, or a mix of hot water, bicarbonate of soda and vinegar, followed by more hot water. Doing this every month or so as routine maintenance prevents it building up again.
Other causes further along the system
An unsealed joint where a waste pipe meets the main soil pipe can let sewer smells into a void behind a bath panel or under a floor, which then finds its way into the room. This is more likely if the smell is constant rather than coming and going with use.
A missing air admittance valve, or a blocked soil vent pipe on the roof, disrupts the airflow in the drainage system and can push smells back through fittings anywhere in the house, not just the bathroom.
A washing machine or basin waste connected badly, for example pushed into a soil pipe without a proper trap or seal, can allow smells to travel back up that connection.
On a toilet, a failing wax seal or pan connector between the toilet and the soil pipe can let sewer gas escape at the base of the toilet. This often shows up as a smell localised right at floor level around the toilet, sometimes with slight staining or a rocking toilet as an additional clue.
What you can safely check yourself
Flush every trap in the room by running water and flushing the toilet, and note whether the smell changes. Clean visible waste covers and remove hair or debris. Check that no fitting nearby has gone unused for a long period. Look for any visible gap or poor sealant around the base of the toilet or under a bath panel, and check whether the smell is worse in one specific spot, which helps narrow down the source before calling anyone out.
When pipework needs to be opened up
If flushing and cleaning every trap does not shift the smell, if it is localised right around the base of the toilet suggesting a failed pan connector, or if you suspect an unsealed joint behind a panel or under a floor, this is the point to have the pipework opened up and inspected rather than guessing further. A toilet that rocks slightly, has ever leaked at the base, or has had its seal disturbed by previous work is a reasonable candidate for having its connection checked and, if needed, replaced.
Your questions answered
Why does my bathroom only smell after I have not used it for a few days?
This is a classic sign of a dried-out trap. The water seal in the U-bend evaporates when a fitting is unused for a while, letting drain smells through. Running water through the fitting for a minute usually resolves it.
I've cleaned the waste and the smell is still there, what next?
If cleaning and flushing every trap has not helped, the cause is more likely to be further along the system, such as an unsealed pipe joint, a missing air admittance valve, or a failed toilet pan seal, and it is worth having this looked at rather than continuing to guess.
Is a drain smell in the bathroom dangerous?
An occasional mild smell from a dried-out trap is not a health risk once fixed. A persistent sewage smell should not be ignored, since it usually means gas is escaping from the drainage system somewhere it should not be, and it is worth getting checked.
Why does the smell seem to come from right around the toilet?
A smell localised at floor level around the toilet often points to a worn seal between the toilet and the soil pipe. If the toilet also rocks slightly or has ever leaked at the base, this connection is worth checking.
Can I use bleach to get rid of a drain smell?
Bleach can mask a smell temporarily but does not clear biofilm build-up as effectively as a proper pipe cleaner or a hot water, bicarbonate of soda and vinegar flush, and it will not fix a dried-out trap or a failed seal.
How often should I run water through an unused shower or basin?
As a rule of thumb, running water through every fitting at least once every one to two weeks is enough to keep the trap seal topped up and prevent smells returning.