The bulb sits in a well in the header normally. There may or may not be thermal paste for improved heat transfer, usually not.
The H100 is the smalled gas fired unit Hayward makes, AFAIK. It has a different design than everything other gas heater Hayward made for the last 15 years. I don't see them much since I primarily work on I/G pools and this unit is meant for AGPs and spas.
It may be getting it's temp readings via contact with the intake, wouldn't surprise me a bit. If this is the case, and you see water, it's likely condensation. Every other gas fired pool heater I have worked has had a well that the bulb sits in, The well makes contact with the incoming water and the bulb makes contact or is in close enough proximity that little heat is lost and the thermostat can make a reasonably accurate reading.
Scott