Definitely call & speak to the property manager. As others have said perhaps play it innocent & ask about his/her intended timeframe for next inspection & if your SIL had any feedback etc.
We rent & have inspections every 6months - and they always give written notice about the intention to do an inspection (asking us to organise a time within the next 2weeks) & we always get a letter within a week afterwards with the feedback.
The property manager & owners (never lived somewhere where the owners came on the inspection but this couple do...) have a quick look around checking for obvious damage & then check if we have any issues etc. And they are always dressed & behave professionally, the PM has a folder with documentation for the inspection that she checks off.
If turning up on days off in casual attire is how their agency is run, it sounds a bit unprofessional to me.