Corrected entry: When The Initiative are looking through the thermal goggles, it shows the first three or four people before Spike to be 68.6 degrees. But not everyone's temperature is 68.6 - it's just an average. Running or even talking could make it a different temp.
Corrected entry: In the scene where they look through the window into the room with thermal cameras, there is a big mistake: Thermal cameras can't see through windows.
Correction: This just isn't true. Depending on the type of glass, windows may or may not absorb or deflect IR radiation (heat). Industrial and military IR scopes/goggles can also be tuned to enough sensitivity to nonetheless provide an image. Moreover, advanced scopes can combine signals from multiple sensors at differing sensitivities in order to allow a more complete image.
Corrected entry: In "The Initiative" in Season 4, Spike is given the chip implant that causes intense neurological pain if he harms a human. We see this take effect when he tries to bite Willow, but he was able to beat up the doctors at The Initiative to escape.
Correction: It's possible the chip was only activated after his escape from The Initiative.
Correction: As you say, that's the average reading, and there's not a lot of variance from that. It stands to reason that a sampling of only 3 or 4 people could all show the same temperature.
JC Fernandez