Had drinks with BFF last night who works in mid-sized consultancy firm, lets call it X,in a big city. She's in junior role and was passed on small task CEO requested from her manager. She completed task and her manager told her to email CEO directly to report on task completed. She sent professional email that started "Dear CEO...XYZ has been completed. Kind regards, BFF".
Other than this email, BFF has never had contact with male CEO, maybe one very brief conversation.
CEO also based in U.K. And emailed back in middle of night "Thank you. This would only happen at X :)".
BFF found it bit odd. Other friend thought just being nice/friendly but probably sent after a drink or 3, but I find it a bit iffy because she said it's very hierarchical where she works and CEO hasn't used it in comms to anyone else on her team. AIBU to advise her to keep an eye out or am I being over wary?