Why were the men who visited the jail so sure that Heck Tate wouldn't show up?

The men were sure Heck Tate was out of the jail dealing with other business because they, themselves, called in the report he needed to investigate. They believed that calling Heck away would mean that Tom would be left unprotected and vulnerable to the lynch mob.

“You can turn around and go home again, Walter,” Atticus said pleasantly. “Heck Tate’s around somewhere.”

“The hell he is,” said another man. “Heck’s bunch’s so deep in the woods they won’t get out till mornin‘.”

“Indeed? Why so?” “

Called ‘em off on a snipe hunt,” was the succinct answer. “Didn’t you think a’that, Mr. Finch?”