In other words - they need "reasonable grounds to suspect" not just a "hunch" or "gut feeling".

They can tell you they have reasonable grounds whether or not they have but by that time they've got your cash, nothing is proved on the spot and you have to argue the point at a later date to get it back, but that isn't the point I'm making.
Indeed I could give many examples. This is a recent one.
A friend of my daughter's arrived one evening in tears. Her car had been stopped by police and she was questioned about where she was driving from and to etc and gave a legitimate answer. He then said the car smelt of cannabis. It did not. He asked was she a cannabis user. She said no.
He said under PACE he had grounds to conduct a stop and search because she was driving in an area with a high level of drug crime and that there was information on the police system that her car had been linked with drug supply. Also the car smelt of cannabis.
He did not however proceed to a full search, but warned her that her car reg would remain flagged and she may find herself stopped again. He asked did she want a copy of the record of stop and search and fortunately she said yes as it confirmed the reasons for the S&S
She was upset. I was furious. The next day I accompanied her to the Police Station. Was told her car reg was not flagged on the system either historically or as a result of the night before and that the officer concerned would be contacted. Within an hour he phoned and rather sheepishly apologised.
He claimed he thought she was driving under the influence of cannabis and wanted to warn her (as a deterrent) that she could be stopped again and prosecuted. He admitted her car was not flagged nor ever had been.
I wanted to pursue it. She just wanted it to go away so we left it.