A driver's van was stolen when he left the engine running when he popped into a shop.
The silver Vauxhall Vivaro was parked in Main Street, Northiam, near Rye, between 6.30pm and 6.40pm on Thursday, July 10.
A Sussex Police spokesman said: "We would like to talk to anyone who saw what happened to the unattended van or has any information as to its whereabouts.
“ It appears to be the actions of an opportunist thief and we would like to remind drivers to remember to secure your vehicles before you leave them, even if they plan to leave them unattended for a few moments."
Any witness or anyone with information is asked to call 101 quoting serial 1252 of 10/07.
Alternatively, call the independent charity Crimestoppers anonymously on 0800 555 111.