What tip does the platoon receive that the field is a bad place to set up camp? "In the field"

The troop doesn't necessarily receive a "tip", but rather a sign. The rain and rising river should have deterred them from making their camp in the field.

Mitchell Sanders made a sound in his throat. He hoisted up the rucksack, slipped into the harness, and pulled the straps tight. "All right, but this much for sure. The man knew it was raining. He knew about the river. One plus one. Add it up, you get exactly what happened."

Note, we are also apprised of the fact that their orders to camp were given by higher authority. Cross, however, didn't question that authority or make any other decision based upon his own opinions of the campsite.

Looking out toward the river, he knew for a fact that he had made a mistake setting up here. The order had come from higher, true, but still he should've exercised some field discretion. He should've moved to higher ground for the night, should've radioed in false coordinates. There was nothing he could do now, but still it was a mistake and a hideous waste.