Help-toilet bowl water level drops when we're away

Have someone flush one toilet while you watch the bowl level on one of the other toilets. If you see the water moving around I would suspect a venting issue.

So I've tried various "tests":
1. when I flush one toilet, the water in a different toilet does have a small ripple. No gurgling in the second toilet, but the water moves.
2. When I flush a toilet, there is a brief "gurgle" at the beginning of the flush. I've been told this means a vent problem.
3. I read I should pour 5 gallons of water into a toilet to test if it is a main drain block -- I did that (twice) and no problem. No back up.
4. I located the vent pipes and both are very clean from outside view, well protected with a metal "roof" and screen....just does not seem at all possible that there is debris from the roof vent.
5. The problem seems to be worsening, because now the water level is noticeably low even after just a couple days.
6. No leaks or anything are visible, at all!
.......spoken to so many plumbers...no one seems to know what could be the problem.....

Yes it is a Kohler? Why? Any suggestions on what is wrong? What is a trap seal? Is that the wax ring? I see no leaks and it would be odd for all of the toilets to be failing at the same time.

Local plumbers are totally perplexed - any suggestions?

Doesn't make sense.

I know.....but I'm not making it up and I've observed this many many times consistently for the past couple of months. it's driving me crazy because I don't even know how to begin to fix it. All 4 toilets on two floors have sinking water levels from 1 to 3 inches below normal water level. All flush fine and fill up fine. There is no sewage stink or back up or visible leak. Could every single one have some toilet bowl defect or partial clog?! That seems so unlikely. I hate paying for roto rooter to come and clean everything out when I'm not convinced that is even the problem - they charge a lot. :(.

I Need super master plumber help.

The reason Sylvan asked if it is Kohler as I see the "Bold look" But never saw the word quality mentioned

I must have replaced over 100 Bold look toilets over the years and installed either Gerber Avalanche or Crane

Ever tried to replace parts for a realto?

Rather then a bold look my accounts rather have a toilet that actually works for many years trouble free

I

The water level drops a little so what. As long as it flushes ok and doesn't run completely dry then why fix something that isn't broken. Wait and see what happens.