One who has escaped. An escaper
Its absence from Webster 1913 is evidence that it is a new word. Formerly escaper was used. That still occurs in newspapers, but I think most of us would actually say escapee.
I say it with primary stress on the final -EE, as with virtually all such words, but before that the secondary stress on the ESC- and a neutral -a, which if you think about it is decidedly odd. Why not like escAPE with an added -EE syllable? The ESC- pronunciation must have arisen by analogy with ESCapade.
The word is notable in that both forms exist, with essentially the same meaning. Usually -er and -ee are paired as subject and object: but this itself is a modern development. The original pairing is of -or and -ee in law terms like lessor, lessee, and got extended to pairs like employer, employee. The latter is a recent borrowings from French: in 1900 you could well have seen it as employé with accent and italics.
Michael Quinion on his "World Wide Words" website lists a huge number of words, mainly nonce-words and many of them presumably humorous coinages too, collected from recent newspapers:
arrestee, assaultee, auditee, auditionee, awardee, biographee, callee, contactee, contractee, counsellee, dedicatee, defrostee, detachee, electee, explodee, extraditee, fixee, flirtee, floggee, forgee, hittee, interactee, introducee, investee, lapsee, mentee, murderee, outee, ownee, phonee, pickee, rapee, releasee, rescuee, sackee, shortlistee, slippee, spinee, staree, tagee, ticklee, trampolee.
Of these, the only ones I would consider as being in serious use, and permanent additions to the language, are dedicatee
and perhaps awardee
But there are some other common words where, as with escapee, they refer not to the passive object but to the active subject: an attendee attends and an absentee landlord absents themself.
The ending comes from French -é, -ée (and that from Latin -atus, -ata), and in both French and English the past participle ending is used for both active and passive verbal senses: I have murdered, I have been murdered. In the adjectival sense, normally a murdered person is one who has been murdered, not one who has murdered.
Here's a speculation. That's with a transitive verb with two human participants: one to murder and one to be murdered. With escape and attend there's usually only one human: you escape a prison or indeed escape from a prison, and only rarely from a boring person, and you attend a conference. So where one of the participants is abstract or inanimate like this, the adjectival form is more likely to apply to the human, the only party of interest. Therefore: an escaped prisoner, not an escaped prison. An escapee is one who has escaped, not something that has been escaped.
Sometimes I think we could use multiple agent suffixes: perhaps escapant for one who is escaping now, escapee for one who has escaped, and escaper for one who habitually or professionally escapes.