They may not necessarily have been checking for drugs - Customs may have been checking for bio risks. For example, here in Australia, only treated wood may be i

They may not necessarily have been checking for drugs - Customs may have been checking for bio risks. For example, here in Australia, only treated wood may be imported to prevent the introduction of pests and diseases. Unfortunately, treatment quality isn't easy to check and if the treatment is suspect, then Customs will drill to see if there are bugs etc. A quick Google search ("Importing wood to the UK") confirms that UK Customs do the same.