Two motorbikes were snatched overnight.
It is thought thieves used a van to transport the vehicles - a red 900cc Honda CBR and a green and black Kawasaki -last Wednesday (October 31).
The Honda was taken from a gated underground car park in Tring between 6.30pm and 8am the following day, while the Kawasaki was snatched from outside a home in Briary Way, Hemel Hempstead, between 11.30pm and 8am. Both vehicles were locked and immobilised.
Police are urging people to call them if they see anyone interfering or acting suspiciously near to parked motorbikes at night.
Officers say to note down the descriptions of offenders and any vehicles being used by them and dial 999.