A spate of smartphone and credit/bank card thefts from gym lockers has been causing serious problems in London, and resulting in many thousands of pounds being defrauded.
The reason we mention it here, is that you may wish to warn your staff what to avoid, and perhaps never to leave their credit or bank cards with their smartphone.
The BBC technology pages have the detail you may need at https://www.bbc.co.uk/news/uk-england-london-62809151.
Personally, I have made sure that as well as heeding their advice, our Smartphone settings do not show the content of texts and notifications when locked.