Water siphons out of toilets

I own a 12 unit, 3 story walk up apartment building. There are 4 suites on each floor. 2 suites on each side of the building have back to back plumbing.
In one of the suites the tenant reports that she will hear a loud gurgling sound and then all of the water in her toilet will suck out. I have been there and observed that there was only about 1 inch of water left in her toilet. She is not reporting any clogging or restricted flow problems. I pulled the toilet and found nothing unusual. I went onto the roof and ran a snake down all of the vents.
The toilet is an American Standard. Any suggestions as to what could be causing this?

I'm guessing that the toilet with the issue is on a lower level. , If so, It sounds like a partial blockage. The pipe fill with water downstream of the problem toilet and then slowing will syphon the water from the bowl

I failed to mention that the toilet in question is on the top floor of the 3 story building.

Bravo50 said:

I failed to mention that the toilet in question is on the top floor of the 3 story building.

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It still sounds like a partial blockage somewhere or blobked vent(i don't know how that could happen, unless something nested in the vent). It has to be a venting issue.

The other thing you may want to check is, how long has this been happening? and if someone on the lower levels is draining a abnormally large volume of water at once