antifreeze mystery

[cavx5]

hi everyone..and the mystery continues..2005 4.4 with 19k miles...this is the third time...dealer added antifreeze because plunger in resovoir was about half way down.. antifreeze added plunger full up position...that was about a month ago...checked today and plunger is half way down...no visible leaks at this time... first time it went to dealer for this issue found leak at water pump replaced about 4-5 months ago..tech said it was leaking and drying on the block before it reached ground..soon after checked and low coolant again, brought it back dealer checked found no leak added coolant said that these cars burn antifreeze..now the coolant level is low again..where in the f*** is the coolant going..is this normal..no white smoke or any signs of leak...please help..rob