It is usual for the current tenant not to be in an apartment when it's being viewed by prospective new tenants. There are lots of reasons for this:
- new tenants have been known to blurt out things they don't like, which may in fact be your things (your furniture and possessions) or decisions (the colour you painted the living room) making the experience embarrassing for you.
- you sitting there can prevent them from imagining themselves sitting there
- you might volunteer information the agent was hoping to hide, such as the noisy neighbours, unpleasant landlord personality, or parking problems on the weekend, that wouldn't be noticed in a single visit.
You don't know the agent or the prospective tenants, so you don't "owe" anyone anything. If you volunteer your information you may indeed cost the agent a commission (or make them show the apartment many more times before getting it) and you may spare this particular set of tenants the trouble of renting this apartment. What you will almost certainly cause is a note next time giving you 24 hours notice to please be out of the apartment between say 2 and 4 pm so it can be shown to tenants. A smart agent would not let the information leak happen twice.
If you would prefer not to have to go find something to do when tenants come to see the apartment, stay quiet. If volunteering everything you know to anyone who might need that information, regardless of your relationship, is part of your moral code, then do it, but understand it will have consequences designed to stop you doing it. Depending on where you live, they might even be more unpleasant than not being home during future viewings.