Toilet Mystery

[4570Man]

I don’t see how a blocked vent would be a constant thing. I could see that possibly ( not likely ) when you flushed another toilet there would be nothing to draw a steady vacuum and consistently drain the water. And a suction strong enough to drain the water should entirely drain the bowl and start sucking air which you’d hear.

 

[the old grind]

Venting is proscribed to keep keep effluent velocity up & prevent suction that can siphon traps. Drawn to some low point the trap will let air into the drain. There'll always be a bit of water left.

The trap is there to hold more water, up to it's weir (rt end of blue line) the vent is to keep it there by preventing pressure/vacuum excursions in the drain system.

btw, I have 1 & 1/2 bathrooms downstairs that I don't use, have to pour water into traps/stools a few times/year, esp if I get a whiff doing my washing. (house is pretty airtight gas heat & HW and can back draft thru' a dry trap) When I moved in someone had left a Solo cup in one vent stack, probably climbed the TV tower to watch fireworks next door from the roof. It was faded to a dull clay color and it took a while to notice it, probably when I watched Dave's show from the roof.

 

[two_bit_score]

Adjust the water spot?

 

[denchen]

My sort of logic....Water is going from the front of the bowl. Is it going over the floor...No. Is there any damp patches...No.. So it must be going down the back part of the U bend. [ruling out dogs]. Is it being sucked out by vent problems....If a vacuum was `pulling` it out, the vacuum would be lost as soon as the water level got to the bottom part of the U bend, and the water level would stay at that level. Is there a gap between the final water level and the bottom of the U bend, ..if yes the water is going straight from the front to the back by a `hole or crack`. That `hole or crack` being at the same level at what the water level settles.

 

[PILOON]

After a period of real cold weather my toilet will often act up.
I discovered that that it was caused by the stack vent freezing or choking caused by the escaping very humid air.
Sinks and showers would be unaffected but since toilets needed a sudden quantity of air a flush acted like a piston and would empty the sink P or S traps.
Naturally your nose let U know when the stack vent was frosted over.

OK, my home is DIY and I only learned later that code called for dual linked stack vents to alleviate that exact problem.
And, no, I left it as is, just open the window for the few times the problem occurs.

Makes me wonder if the OP's problem might not be linked to a stack venting issue.