Something to consider, having "hunted" humans with dogs and FLIR in my past; since thermal reads heat, crafty humans would try to hide under and/or cover up their radiated heat to avoid detection from overhead FLIR. Now, with the beaver,consider this possibility: beaver fur is waterproof, for the most part, to protect the animal from body heat loss to increase survivability, especially in their natural water environment since water draws heat away from a mammal to the water , hypothermia. Furthermore, since their fur is dense and water repellent to conserve body heat, it is safe to hypothesize that the very nature of their evolutionary design, drastically reduces heat loss=less thermal signature. Now, add in the fact that most of their body is submerged when in the water, even less heat escapes to be detected by thermal. Water is not a great transmitter of heat, therefore, I'll postulate the beaver has very minimal heat loss to detect and they are very adept at avoiding predators, which you are, by avoiding significant body exposure (heat/scent). Some scent will rise to the water's surface by gassing off (think drowning victims) and some heat transferred to the water by the exposed body (once again i.e.,humans, since we have no fur to protect us and can cause some degree of heating water around the body when the body is alive, recently deceased, or decaying). Therefore, most likely they are radiating very little heat to be detected. Good luck. Keep posting to update your luck at thermal detection.