As the title suggests, what do you expect to see in tech notes? Are your expectations set to include precise details, or is a close-out with an "issue resolved, power cycled "X" unit and system was up-and-running" enough?
During my time as a tech/installer, I would fall on the more detail-oriented side. I'd include the contacts I met/spoke with, the hardware (with model #'s) I interacted with, and troubleshooting steps I performed or, if on a project, how many devices I had installed, where, and how many were left to go.
I ask because there were people before me that left the company, and the notes gave me little info about issues that were particular pain points, and no inclination on how best to prepare for a visit (should I bring XYZ device to replace, what wiring, etc.,), and I would end up with a stuffed van for something that required only a small power supply after a site has a surge or a reader went down that I didn't have, thus increasing time on a task where I could have resolved in 1 trip. I'd make good relationships with contacts that knew me and knew I could solve an issue quickly, and having pertinent info beforehand was always a boon.