This is why linguistics breaks things down into more than just active and passive. "Middle voice" is just one of the many terms out there that describe this, according to Wikipedia:
Patientive (S = O) ambitransitives are those where the single argument of the intransitive (S) corresponds to the object (O) of the transitive. For example, in the sentence John (S) tripped and John (A) tripped Mary (O), John is not the person doing the falling in both sentences. Likely candidates for this type of ambitransitive are verbs that affect an agent spontaneously, or those that can be engineered by an agent. English has bend, break, burn, burst, change, cool, enter, extend, fall, frighten, grow, hurry, melt, move, open, spill, stretch, trip, turn, twist, walk, and many other verbs.
Verbs of this class have been called unaccusative verbs, middle voice, ergative or anticausative verbs in the literature, but again, these terms are not universally defined.
Also to answer your titular question, this is unmistakably passive voice without be:
Best Answer
If a clause has all of the following, then it is in the passive voice:
Example: The documents were printed.
Optionally, the agent is expressed in a prepositional phrase with by: The documents were printed by the printer.
There are some exceptions; though, generally speaking, if a given clause meets all the above conditions, then it is certainly passive voice. The Wikipedia article about the English passive voice has a pretty complete coverage, detailing all cases of English passive voice, but the major exceptions are these: