He doesn't need to PM anyone, he could just place his cursor on the padlock emoji, and the person who locked it between the OP and the moderators will appear. But this happens on the laptop, I've not tried it on the mobile phone.
Nwada001 has posted about it already and also PowerGlove quoted him which are about how people can know if the topic is locked by someone that created the topic or a moderator. Try to read each posts before posting.