Toilet is 25 year old kohler, is it time to retire it

Does the level drop in the bowl, or the tank? If the level drops in the tank, you should only need a new flapper. If it drops in the bowl, it could be a defective bowl, or could merely be something in the trap that is allowing water to wick out of the trap. If bowl level is dropping, I would start by purchasing, or renting a closet auger, and auger the toilet thourally. However, standard toilets are quite reasonably priced, and pretty easy to change out. If your looking for a reason to update the toilet, just use this as an excuse. The newer toilets flush well, and conserve much more water than your current throne. Please let us know how things work out.

I pretty much agree with MGD but water doesn't wick out of bowls, it siphons if your vent is clogged and you will not reach the vent stack with a toilet auger. First answer MGDs question, tank drops or bowl level drops. Rarrely can a toilet not be brought back to life for under $50

I agree with both experts. Time to retiree the old toilet and send it to Florida where old toilets go when they're no longer usable.
Good luck, Tom

Bob, if something like a paper towel, or hand towel got dropped in the toilet, and is hanging up in the trap and over the outlet of the trap, it will wick water out of the bowl. Kind of like a wick on a lantern will wick karosene up to the top of the wick. Not a common occurrence by any means, but it is plausible, especially if they have small kids that may have flushed something that would cause this. Closet auger is a good place to start in this case.

MGD can't say this is impossible just because I have never seen/heard of it but I have seen/heard of toilets siphoning quite a bit if the vent clogs.

I've never run into wicking either. Seems to me if a rag were caught down in the trap paper and solids would hang up and overflow the bowl.
In my book there are only several things that could cause the bowl to lose water.
1. A blocked vent that suctions water out of the bowl. **OR**
2. Sometimes in casting the bowl there is a small bubble in the china. Instead of junking the bowl the manufacture will put a porcelain patch over the hole and send it on through. On occasion the patch will fail allowing the water to seep out of the bowl. If that's the case you would have a full bowl after you flushed and the bowl filled but some time later you would notice a drop in the bowl level. The only other thing might be if you dropped something in the bowl and cracked it. If you're worried about where the water's going, in both cases it drains back down into the sewer and not under the bowl on the floor. To check the bowl must be pulled, filled and put up on saw horses. You can then see if there's any leakage from the bowl.
But why go to all that trouble?
Simply replace the toilet. Good luck, Tom