Are house elves invisible at times?

[Sandra Collins]

No: HPFGUIDX 136205

I was just wondering. In the HBP, Dobbie and the other House 
Elf (the name escapes me) follow Draco to see what he's up to - 
neither of them are particularly intelligent and I wouldn't class 
either of them as being on the Sherlock Holmes level of 
investigating, so to avoid raising Dracos' suspcions (or those 
around him) does it say anywhere in the books if House Elves 
ever invisible?  They can apparate all over the place as shown by 
Dobbie when he appears at the Dursley's place, and in the 
hospital as well, in one of the previous stories. So do their 
magical powers extend further, because I reckon Draco would 
have spotted an ex-Malfoy House Elf acting suspiciously.
Just a thought!

Sandra