An appeal has been made for anyone who may have seen someone suspicious in the area to contact the charity.
Humberside Police and BT have appealed for anyone who saw suspicious activity in the area to contact the force.
The men, aged 33 and 30, were arrested after a suspicious vehicle was seen in the Cullompton area on Monday.
Greater Manchester Police said people should exercise caution while walking their dogs in the area near the reservoir and report any suspicious packages.
As authorities searched the scene, numerous suspicious packages were found, possibly because people fled the area, leaving items behind.
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After a sweep of the area, Coast Guard officials found no sign of anything suspicious and reopened most of the harbor.
Officers will be stopping motorists, cyclists and walkers in the area over the weekend asking people whether they remember seeing anything suspicious last weekend.
Detectives have appealed for anyone who was in the Upper Cranlome Road area at the time of the incident and who saw any suspicious activity to contact them.
If you receive a suspicious package, don't open it, isolate it, evacuate the area around it and call police or a postal inspector.
Police closed the area around the Psychiatric University Clinic, about 500m from the gallery, on Monday after a suspicious car was found.
"We are still keen to hear from anyone else who was in or around Gloucester Street in the North Laine area of the city centre on Thursday evening or the early hours of Friday morning and who saw anything suspicious, " said Det Insp Carwyn Hughes.
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