I don't even like to the say the "M" word. As @Dawn Brenengen pointed out, iterations of mold is all over the place and my understanding is that very few of the cultures are actually detrimental to typical humans. The real challenge becomes when you have either a super sensitive tenant to any traces...or a super psychologically sensitive tenant that is quick to flex the tenants rights and demand this and that. Owning a management company we have to confront this issue pretty frequently. I like to use the reasonableness test in these types of situations. If a tenant reports a smell, that triggers an observation. If you see "M" or mildew, it could be treated on the surface with bleach or other mitigating solvents. If you don't see it or even smell it yourself then you have a decision to make. If cleaning the area doesn't satisfy the tenant then another alternative is the mold remediation company. Of course that call is like poking the hive that has now become a whole cottage industry with test-in's, abatement, removal, construction, test-out's...it can be ridiculously expensive. The worst is when you receive the initial $150 test that they offer and it is inconclusive but there are particulates that are of such characteristics that "M" cannot be ruled out. Well thanks for nothing. Now you have to upgrade to the more thorough test and of course they are more likely to find traces of "M" in any of the wet/plumbed areas of the unit. Apologies for the venting...I digress.
The real issue in my opinion is making sure you track down the source. Water intrusion or a plumbing leak can be a latent killer that can lead to major wall and sub-flooring repairs if not identified soon enough. If you do see some signs of "M" or mildew it is at a minimum a sign of trapped moisture. The most common examples we all see is the poorly ventilated bathroom without a fan or a window...combined with long steamy showers.