Marian Halcombe's stupidity

1/ Burns Walter's letter for fear of it being read by someone else, and writes about it in her diary?

2/ Realises that Sir Percival is not the honest, courteous, charming man he previously appears to be, believes that there's something suspicious in the Anne Catherick business, knows that Mrs Catherick doesn't want him to know she has been to the house and knows that she has to investigate the reasons, and yet talks about the dog and lets Percival know Mrs Catherick has been there?

3/ Writes a letter in secret and knows she shouldn't cause suspicion (which she warns Laura herself) but doesn't seal it and then places it in the post-bag where she can be seen, and doesn't suspect it when suddenly Countess Fosco wants to speak to her?

Even the fact that Marian doesn't try, in some ways, to find out where Percival goes and what he does in such a hurry, is quite stupid. She knows that it has to do with Mrs Catherick and Anne Catherick, and though there's no proof of Percival having done anything wrong morally or legally, his change in manners, the conversation Marian eavesdrops and some details here and there unveiling part of his true character, should lead her to think more about Anne's letter and wonder how true it is. Perhaps she cannot carry out the investigation herself, but at least should put on her guard and be careful with everything she does. 

Of course these actions are more or less justifiable (e.g. concerning the dog, several people surround her and get stressed out about the blood and ask her repeatedly and force her to blurt out whose dog it is), and they are meant to advance the plot of The Woman in White, but they're nevertheless irritating. Why so slow, Marian? You're supposed to be the intelligent, sharp, independent one. 



Update at 6.30pm: 
I wonder how long it will take the "intelligent" Marian to realise that the Count, with the help of the Countess, spies on her and Laura and apparently sides with them against Percival only to gain her trust. 

Update at 9.40pm: 
The 2nd time Marian has to send letters, she takes care to give them to Fanny instead of placing them in the post-bag, and looks around to check everyone's whereabouts. Then she goes into the house, and goes out again to go to Fanny's place, this time not knowing where others are and not checking. 
Later she rises a bit in my esteem by carrying out some investigation, i.e. following Percival and Fosco, but disappoints me soon afterwards- why continue writing in the diary when not well? At least she should lock the door once inside her own room. 
Her carelessness now appears like a bit of a farce. It becomes a necessary tool for the advancement of the plot, and it is absurd.