Some states and localities require the consent of both parties to the recording for it to be legal. Check in your area. That being said, in a public place an employee has no reasonable expectation of privacy so even if detected you are unlikely to be hauled into court.

I am a big advocate of recording because in case of doubt whether an employee actually said something they were supposed to or not, I can refer to the recording and give them credit for it if they are due. We go through life not memorizing exact verbiage of greetings, presentations and farewells but rather the 'meat' of the interaction--the impression that it was friendly or persuasive. Reports often need a lot more than impressions.

I am a big advocate of recordings because sometimes multiple timings cannot be done in person unobtrusively. The recorder allows you to recapture the times at home by elapsed time on your recorder between sound points.

I am a big advocate of recordings because if you do 5 shops or 10 shops between performing the shop and reporting it, there is a lot that can be forgotten even if you took decent notes after each shop.

My own equipment is an old Sony ICD-MX20 that I don't know has been made for a few years now. I have bought backups of it on eBay in anticipation that one of these days the recorder will die on me, but it just keeps going. It has a connection to my computer through the USB port so sound files can be uploaded and stored with the other materials for the shop should I need to go back to them later. I am using a Sony condensor microphone as the external mike. It is a lapel microphone. My current one is, I am sure, a knock off as it was about $1 + $10 shipping on eBay (my original genuine Sony was close to $50).

DVRs have gotten a lot cheaper since I bought mine. Some now plug directly into your USB port rather than requiring a cable to do so. I understand Radio Shack has lapel microphones in the $20 range. My first recorder was a cheap little Olympus that had no computer connectivity and no possibility of an external microphone. Playing back recordings was a PITA and the sound quality was pretty bad because the recorder had to be kept out of sight in a pocket or purse and generally sales people talk towards your face rather than your pocket or purse. The external microphone allows sound projected towards my face to be picked up while the equipment is still out of sight in my pocket.

The purpose of recording is for MY use in preparing a report as accurately as possible. If I were recording on behalf of an MSP to submit the recording, I would be much more cautious about state and local laws.