This is a 'Trail and error' job. Take off the side panel of bath and check the state of the floor under the waste and all its joints also check the taps and feed pipe joints. That will rule them out if dry.
Check the same with the basin. The hard part is checking the shower waste if you have a purpose shower unless there is a panel you can remove to access the waste. check out the feed pipes if possible of the shower to make sure the're not leaking.
Doubtful, but could the smell be coming from the hot water pipes if they are plastic? as you say it comes and goes.
If the both water feed pipes are plastic then check if the smell is around and stronger when you run hot water. If all produces a no it's not that lot then I'm afraid it time to lift the flooring and see whats under there that might be causing it. I hope you haven't upset any tradesmen that have worked on your property!! they may have put a fish under the floor!!!
BOB
February 2005