Where they are
German cockroaches want warmth, water and food residue. The motor housing of a fridge, the space under a dishwasher and the void behind a cooker are the usual harbourages. They stay hidden by day and come out along the skirting at night.
Why a count beats a sighting
A sighting tells you they exist. A count tells you how many are moving and whether the number is going up. A monitor along the skirting draws them with a scent they already respond to and counts each one that enters, all night, without you standing in the kitchen.
Small ones are the signal that matters. Nymphs mean breeding, and breeding means it is time for a proper treatment rather than a spray from the supermarket.
After treatment
Leave the monitor in place. Counts should fall to zero within a couple of weeks. If they do not, the bait or the harbourage search needs revisiting, and you will know that from the number rather than from the next sighting.