The stay behind chaps had massive bunkers to hide in, and lots of the planned defensive positions were pre dug too. Iirc the main preparation of bridges were ready built chambers for the insertion of demolition charges.
A bunker next to a bridge would be fairly obvious, the positions were concealed. My pal Lutz was in the Bundeswehr, he said he spent pretty much all his duty time in a bunker.
"Mistakes in the initial deployment cannot be rectified" - Helmuth von Moltke