Thank you everyone for your thoughts. I'm thinking it might be OT cries, as you have mentioned, when she's transitioning between sleep cycles. She has never slept as soundly at my parents' house as she does at home, so would make sense I think that it happens more often there.
HenaV, the cry outs began shortly before we cut out her morn nap (but her naps were compromised then as she wasn't taking two proper naps anymore), so perhaps still due to being OT? We just moved up her nap to noon this week, so maybe some time until she adjusts. She's definitely exhausted in the morning these days. And yes, it only happens at nap time.