It's the law in Thailand, not mentioned in travel brochures

Police Stop & Searches Around Sukhumvit / Asoke On The Rise

Let’s get authoritarian | Bangkok Post: Post Bag Opinion

Everyone is required to carry identification.  In case of foreigners, the only acceptable identification is the passport.  The few times I have been stopped while driving a motorcycle, a Thais drivers license was sufficient.

Thai law gives the police the authority to stop vehicles and pedestrians and demand identification, ask to see the contents of pockets, and bags.  They can also demand a urine sample.

Drug use in Thailand, especially yaba (methamphetamine) is quite rampant. 
The reasoning for these searches is to apprehend drug users and dealers.  Besides apprehending drug transporters, stopping vehicles also leads to discovery of unregistered vehicles, unlicensed drivers, and stolen vehicles.

The best thing is to be aware of these laws and carry proper identification and follow the police officer's instructions.  Don't give the police a reason to arrest you or 'fine you.'

Urine tests are most prevalent in areas predominant with bars and clubs.
It was about 15 years ago when I had to submit to a urine test.  It was done at a disco, the place was shut down and about 200 people had to submit to a urine test.  I wrote about that before on January 8, 2011 Amazing Thailand: Enjoyable Recollections

Last year I heard of random urine test being done on the Phu Phan highway. 
You can also expect vehicle searches for drugs on highways and streets near Thailand's borders.
Sakon Nakhon