A Florida man was taken into police custody last week ... and cops made a most unexpected discovery jammed all the way up his butt -- a full-size thermos! -- but that's only half the story.

TMZ has obtained exclusive 911 audio from the sh---y situation in which a caller reports there's a naked man in a restroom at a public park who had "been there all day and won't leave." Polk County Sheriff's Office

TMZ has learned sheriff's deputies showed up and found him fully clothed ... they initially gave the guy -- identified as Walter Frymire -- a break, escorting him out of the Winter Haven park with just a warning ... but later, Frymire allegedly trespassed again.

This time, a Polk County Sheriff's deputy arrested him ... and here's where it gets really weird. Every arrestee is reported

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