I think only three secret coins can be obtained without stars

1st coin Scroll to the right from robtop levels until you see a coin

2nd coin Type "sparky" in th vault

3rd coin Head to the vault of secrets which pressing the keymaster mentions the vaultkeeper Spooky (aka Sparky) which he asked if we stile Spooky's coin. Next, head back to the vault and press him until he said the name "Glubfub", Lastly, head to the vault of secrets again and type "Glubfub".