nicktheplumber
RP is correct about the bowl crack or siphonage as likely causes of such a rapid loss of bowl water. Another possibility is negative pressure in the drainpipe. This is rare but can occur with high velocity wind over the stack vent. This causes the water in the bowl to drain out due to difference in atmospheric pressure between the bowl water surface and the waste line. I’ve heard of it, it makes sense, but it is apparently very rare and I’ve never seen such a case.
I have also seen only a few cracked toilet bowls. They usually manefest with leaks onto the floor that you can see in the bathroom near the bowl. If you can get under the bowl (as in a basement) look for dampened floor boards. The solution is to replace the toilet.
The most common cause of what you describe is siphonage due to something (especially a rag) that is hung up in the trap. Usually part of the rag hangs over the trap outlet and “wicks” the water into the drain. The best solution is to run a closet auger through the bowl and pull out the rag… I’ve seen that at least a dozen times, so in my opinion it is the most likely culprit.
NtP
