A Noise Upstairs

Randy is a nurse in Ambulatory Care.His wife Addy is a clinical specialist in Rehab.They recently bought their first home.They are expecting their second child and were delighted to find a nicely kept 4 bedroom split level.They could keep their office at home and each child would have a room.

The house was on a wooded lot in a new development.With the usual massive undertaking of a move they were settled in within a week.
Excited as kids they sat in the downstairs den to share some champagne in celebration of unpacking.Their three year old daughter Megan was asleep upstairs in her new room.

As they snuggled on the couch,They heard a thump on the floor above and the sound of blocks being thrown around out of the toy box no doubt.With a sigh and "Oh No, Guess who woke up."Addy went up to check.She soon returned and said "That is strange,Megan is still in bed but there were blocks all over the floor.I thought you cleaned up before you put her down tonight,"

"Well I did, you know you checked the room before we came down here.No blocks were out."snapped Randy.
"Don't get bent out of shape"replied Addy."No big deal."

Three days later,Addy was home alone.While downstairs washing,she heard noises upstairs sounding like a child running and laughing.Randy teased her when he and Megan got home.
The next night while Addy was at a friends house,Randy and Megan heard the same thing.Megan clapped her hands and rushed upstairs saying "Sissy,Sissy.play."
Two nights later they found Megan sitting on her bed as a ball was bouncing around the room.She told her astonished parents that Sissy came to play a lot.Sissy had no Mommy so did not go to bed....

Is Sissy an imaginary playmate?Doesn't sound like it to me...