They seem to appreciate it and, as far as I can tell, don't consider it intrusive.

You can ask questions, in a way that is not too intrusive and asked showing genuine concern.

It was a pain to prevent water intrusion.

If they're intrusive at all, forget it.If you have too many rental units or too many owned by one entity, the project can become non-warrantable, making it harder to finance.You're competing with apartments.Honestly, I rarely consider condos.

Are there now mold issues or other contamination due to the water intrusion?

I know in Puerto Rico, while there is desire to invest and fix it the way the investors want - the local governments are wary and can become overly involved and intrusive.

If not then you can just run the pump when the rain comes and maybe find the point(s) of intrusion and do something to prevent more water from coming in like caulking any cracks with silicone.

You normally have to remove 4ft of drywall & insulation in all directions from where mold is found.It's no big deal if you can  eliminate the water intrusion, cut out the sheetrock, and replace it.   

I think if you were to door-knock it may feel a bit intrusive?

So a pocket listing can be a very non-intrusive way to test the market without the hassle of officially listing.It also gives the seller a much stronger negotiation position.