He told me that he had a temperature measuring device, that he could not find, back in 2011
He needed something like a data logger. Easy and light, cheap, with a battery lasting over year or so, that he could install at customers' places in order to have a Log of "what happened" if any problem came up.
Say, a frozen (broken) pipe or other malfunctions in heating, AC and such. He said connectivity wasn't necessary and to log to a SD-card that could be inspected later would be sufficient.