According to latest test results on Geekbench test by Primelabs, Android 5.2 was found running on Nexus 5. Earlier to this Nexus 5 was found running on 5.1
Geekbench clearly showed results on both, Single Core and Multicore results. Earlier rumors mentioned 5.1 will be released in February, but Android 5.2 has taken every one with surprise. Still 5.1 has to be rolled out on lot many devices. Initially it was Moto followed by few more manufacturers like Lenovo , Nexus and Android One.
One possibility , may be Google will be in the process of finalizing a new version in the M series in continuation to Lollipop to counter New IOS.