Nothing surprising.
Vader had been hunting the rebels for a while and would expect them to be hiding on a remote system which would be virtually uninhabited, since otherwise they would have found them already. The Empire know pretty much what is going on and where. The probe report of advanced constructions on an ice planet is completely unexpected and certainly suspicious and worth inspecting: something's going on in there that was meant to hide from the Empire.
Masters of the Force can sense each other through the Force. Luke was on Hoth, so, Vader gets an additional reading that implies a Jedi is involved, which seals the deal.
This storyline (the higher the level Force mastery, the clearer the sense of presence) is used again on Return of the Jedi: Luke's presence endangers the whole rebel mission and in fact was key in the Emperor entrapping them in the final battle. Luke is a master on his own, yet still an apprentice in comparison with the Emperor, who is a master of evil plans. He knows their whole plan just using his sense. Luke is learning and has just grown strong enough to sense that Vader was on Endor at the same time with them. This implies also that the Dark Side of the Force is stronger in sensing... like a predator can sense his pray.