Asked whether the police felt that the issue had damaged their investigation, the spokeswoman said: "We don't know because we don't know what was on the phone."
Other police forces affected by the issue include:
• Cambridgeshire - one incident between August 2013 and August 2014
• Durham - one incident during the same period
• Nottingham - one incident
Microwave help
Mr Munro, who analyses hundreds of laptops, tablets, phones and other devices for corporate clients, said: "When we seize a device for digital forensics, we put it immediately into a radio-frequency shielded bag, which prevents any signals from getting through.
"If we can't get to the scene within an hour, we tell the client to pop it in a microwave oven.
"The microwave is reasonably effective as a shield against mobile or tablet signals - just don't turn it on."
SecureDrives, which develops hard drives for the military, is releasing one next year that can be physically destroyed just by sending a text message.
The hard drive -which will cost more than £1,000 - is also immune to the radio-frequency blocking bags.
"The hard drive is constantly looking for GSM [Global System for Mobile Communications] signals, if it is starved of them it it would destroy itself. It would see such a bag as a threat," said James Little, head of sales at SecureDrives.