An engineer had a sat nav and hundreds of pounds worth of power tools stolen from his van.
He had left it unlocked while working in a house in Nettleden Road, Little Gaddesden, between 9am and noon on Friday.
Sgt Peter Huffer said: “If you are working from your van, make sure that you lock it every time that you go backwards and forwards, because you may be being watched.
“Thieves will act very quickly to take your power tools. Be vigilant at all times.
“A criminal will very often go to different areas to work where they are not known. They will travel for miles and in rural areas, there’s less likelyhood of them being seen.”