Walter's shirt has sweat patches front and back after being in the hot chimney. Once outside among the cars his shirt has no damp patches. With a lab door between Walter and the perpetrator in the building Walter's damp patches become more visible. And when Walter is outside a main door the damp patches disappear. Nothing dries that quick.
In the opening scenes a technician is weighing samples in a containment lab, and calling out the weights in ounces. Labs use metric measurements for everything, so he should be calling out grams.
A strong nylon rope would not have melted in the ambient air temperature of the chimney. It would have been prudent to have one on hand for either of the chimney escapades.
All marked patrol cars have a metal screen or "cage" between the front and back seats. This prevents people from climbing from the back to the front as well as preventing attacks on the driver from behind.