[quote=Dervish]
 I can't imagine the salarians allowing one to escape. 
[/quote]

[url=http://masseffect.wikia.com/wiki/Yahg]To be fair, Mass Effect 3 does say it happened.[/url]

[quote]A salarian research facility on Sur'Kesh captured and experimented on several yahg. The salarians planned to secretly uplift the yahg and use them to further their initiatives with full deniability in a similar manner as the krogan. During a Cerberus assault on the STG base, one yahg breaks out of containment and goes on a rampage, killing several Cerberus soldiers. Its fate is unknown.[/quote]

Though, looking at it again, the involvement of Cerberus/the way it's read makes it seem like it was during ME3 or slightly before.