From the Mackinac Michigan GenWeb site (Captain Snelling's activities before General Hull surrendered Detroit):
Miller sent a spy into Brownstown and understood from Captain Maxwell that the enemy had disappears. He then returned to the battle field to collect his dead and wounded and encamped there for the night. Next day Miller got the wounded into boats, and thinking that the communication with Brush on the Raisin had been opened, and failing to get sufficient supplies of provisions for his men, although he had sent Captain Snelling to Hull for that purpose, he took up his line of march on the 11th for Detroit, reaching there next day.
On the morning of the 15th Brock opened his battery, which was followed by a flag of truce, received by Captains Charles Fuller and Josiah Snelling, 4th U. S. Infantry, with the demand for surrender, coupled with the threat that if not complied with he could not control the Indians then under his command.
In the evening of the 15th movements of the enemy indicated a crossing at Springwells by the collecting of boats and bringing up of British vessels. Captain Snelling, with a few men and a field piece, had been sent down to the sand hill, nearly opposite Sandwich, to reconnoitre and to watch the crossing, with orders to return and report before daylight.
Gen. Hull sent Captain Snelling with a note to Gen. Brock, which he delivered; the purport of which was that he agreed to surrender the fort. Col. McDonald and Captain Glegg were sent by Gen. Brock to agree upon the terms of the surrender. [Source]