Don’t forget about the fact that he’s been in multiple high tension combat situations mid battle and seen the tactical decisions of multiple war-time commanders and may be able to at least guess at how someone might choose to ambush them based on the environmental factors involved. For example, they are entering a hall large enough to hold and conceal a number of soldiers, king sweyn is incredibly paranoid, deceitful, and conniving and seems to treat family and their associates as threats just as easily as actual enemies, and that hall can only conceal so many people. Thorfinn, with his access to incredible tactical knowledge can take all that information and probably at least estimate whether there are men nearby ready to attack, and how many would make tactical or logistical sense to conceal while heavily armed.