A homemade bomb has been found in front of Wat Or Noi, where protester Luang Pu Buddha Isara is based, in Kamphaeng Saen district of Nakhon Pathom.
Pol Col Sombat Ornsomboon, chief of Kamphaeng Saen police station, said the bomb was probably left there on Sunday night.
It was safely detonated by police yesterday.
Pol Col Sombat said he thought it was possibly a threat to Luang Pu ahead of his planned return to Wat Or Noi yesterday.
The monk was due to stay at the temple last night and return to his protest base on Chaeng Watthana Road today.
Meanwhile, police said they still had no leads on who was to blame for the grenade attacks at the anti-government protest base controlled by the monk on Sunday night.
The incident injured 45-year-old guard Somboon Thongnate.
Pol Col Kamtorn Ouichareon, chief of the Explosive Ordnance Disposal unit, said police investigators could not yet determine where the two M79 grenade rounds were fired from.
One hit the People's Democratic Reform Committee (PDRC) stage.
The other landed inside the compound of the Anti-Aircraft Artillery Division nearby.
The PDRC stage in the area has been the target of M79 grenade attacks on several occasions.
Lung Pu believed the grenades on Sunday night were aimed at his temporary residence on Chaeng Watthana Road but missed their intended target.
Two grenades were also fired at the rally site of the Network of Students and People for the Reform of Thailand near Government House on Phitsanulok Road in Bangkok on Saturday night, and another one at a Rangsit University building in Pathum Thani on Sunday.
Former Election Commission secretary-general Pirun Chatwanichkul called for serious efforts from police and soldiers to stop the grenade attacks.
"The attacks could be stopped if intelligence officials do their work,'' he said.
"It should not be difficult to find out who is behind these attacks, but security authorities have no measures in place to prevent the attacks at all,'' Mr Pirun said.