he was seen playing by me

see sb do sth（表示看到全过程） 和see sb doing sth（表示看到事情正在进行）是两种不同的固定搭配.
see sb do sth表被动时还原to为sb was seen to do sth
see sb doing sth表被动时为 sb was seen doing sth
he was seen playing by me（我看到他玩了）强调他正在玩.
该句使用的是第二种句型.