Fire crews were called to a series of small blazes at a garage block in Hemel Hempstead over the bank holiday weekend.
Police were also informed of the two rubbish fires in two days which were reported in a garage block in Windmill Road, Adeyfield.
The first incident happened at 8.24pm on Saturday while the next was dealt with at 5.27pm on Sunday.
In both cases a pile of rubbish had been burnt, but there was no damage to the garages. Firefighters from Hemel Hempstead Fire Station tackled the flames using one hosereel.
Both fires were believed to have been started deliberately.