Searches started in the area of Faebait Farm near Muir of Ord on 15 March after the farm owner’s dog become unwell and later died.
Police Scotland said tests had found Aldicarb – a pesticide that has been banned for more than 10 years.
Officers do not know if the substance was spread deliberately or if someone has used the area to discard the pesticide.
Walkers are being urged to report any unusual items in the area but not to touch them.
Inspector Mike Middlehurst said: “Due to the time since the original report the substance will now have degraded substantially into the soil, particularly given the recent weather conditions we have had.
“We are awaiting some tests back from our colleagues at SASA (Science and Advice for Scottish Agriculture).
“I would still advise dog walkers to ensure that pets are kept on a short lead and under close control when walking in the area.
“Any person who comes across any items that cause them concern should contact the police immediately. Do not touch the items.
“I would like to thank all the property owners in the area for their full co-operation and frankness in their passing of information regarding this incident.
“Should anyone find substances in a shed or outhouse that they are not sure about, please contact your local Scottish Government Rural Payments Directorate office for directions and instructions as to what to do.
“Do not disturb the items or take them into a warm environment or transport them as you will not know what consequences this could have.
“Anybody with information about anyone possessing, storing or using banned pesticides are urged to contact Police Scotland on 101 or Crimestoppers on 0800 555 111.”