Smells are a good indication that something is amiss. Use your senses and do some investigating. If this is something you are not comfortable with or cannot find then you should call a plumber. When it comes to plumbing smells it is always best to be safe, so don't ignore something just because it seems minor. You should remember as well that if you are going to call a plumber make sure you have one that is nearby you. For example, if you lived in Bournemouth you should get a Plumber in Bournemouth, sounds simple enough but it's very easy to forget this sometimes!
Sewer gas smell
This is really one smell to look out for because it can also be bad for you to breath. Most fixtures have built in protection from sewer gas leakage by a water trap. If the drain trap dries out then you will smell sewer gas escaping from the drain line. When you begin to smell this investigate right away. Run water down all of the fixtures and see if this takes care of the problem first.
A moldy smell usually means moisture. This might be a slow leak somewhere. Check first in all of the easy access locations like under sinks and behind access panels. Failing that you may have to go for a crawl under your house (or attic if that's where your pipes are) if possible to see if there are any visible signs of leaking.
Natural Gas or Propane Odor
If you are smelling gas this can be very unsafe as natural gas is highly explosive. Investigate any signs of a gas leak and call the gas company, or a plumber right away.