Well, you don't know much about kobolds, you haven't seen many roaming about, and you haven't travelled that much yourself, outside of your days in the Service. So you're not sure why he spoke up. Or to put it bluntly, you failed your Insight check. Still, you did ask Zimmer to look for volunteers.

As for an ambush, that seems exceptionally unlikely. There's little cover, a few copses of small trees that could hide a man or two, if they were competent, but other than that they'd have to either be on the ridge, like you, and therefore pretty exposed, or else hiding in the floodwater or in the river. Could be lizardmen, maybe, they're a prominent part of Moolyager folklore, but those stories mostly predate the Empire.