No. This would not be illegal.

You are not trespassing or breaking and entering since you have permission to be on the premises, and you are the rightful owner of the guitar so you are not depriving him of property that belongs to him.

If you did this with the assistance of a law enforcement officer, rather than Bob's family, without a court order describing the property to be retrieved, this would be called a "civil assist".

If the guitar were collateral for a loan, it would be a self-help "repossession" (a.k.a. "repo") authorized by the Uniform Commercial Code if it could be accomplished without a breach of the peace, and it would not require a court order.

If you did this pursuant to a court order the case you brought to retrieve the guitar would be called a "replevin action" and you would also need to obtain a "writ of assistance" to authorize a trespass in the presence of law enforcement to retrieve the guitar.

Also, Bob would still be be guilty of the crime of theft of the guitar, even though you got it back, because he took it with an intent to permanently deprive you, its owner, of the property that belongs to you. You could also probably sue him for conversion or "civil theft" in some jurisdictions, but your damages would be nominal except for punitive or statutory damages under a civil theft statute, because you ultimately got the guitar back, and so you suffered only minimal economic harm.

Ohwilleke的回答很好，但還有一點需要考慮。

您說Bob是您的前任。他是否有理由認為這對吉他是您一對夫婦共同購買的，因此他有權使用它？如果可以聯合購買，那麼您就不會偷回它，但他也不會。您可能最終因擁有吉他而被起訴。如果這是共同購買的商品，您可以考慮將他的一半錢支付給他。您能證明吉他是您真正的吉他嗎？如果可以，或者他們知道吉他是您的，那就太好了。如果沒有，您希望在他們交出之前說服他們。 （我之所以說“希望”，是因為他們天真地愚蠢地拒絕了。）當然，他們也有可能打電話給Bob進行檢查。

如果對誰擁有吉他存在爭議，您可以起訴他這並不一定聽起來那麼痛苦-在英格蘭，我們設有小額錢債法庭，專門為此類案件而設計，雙方的費用均最低，但您所在的國家可能會有所不同。

In England and Wales, it is possible that you could be found guilty of theft. The "property belonging to another" includes where somebody has "posession and control" over somebody else's property.

This was shown in R v Turner (No 2) [1971] 1 WLR 901, where a defendant was convicted of stealing his own car from a garage where he had left it for a service. It was considered that, since the car was in the control of the garage, it was property belonging to them for the purposes of s1 Theft Act 1968.

在德國，根據 Art，這很可能是非法的。 858 BGB：

（1）違反所有者的意願，剝奪所有者的佔有權或乾擾所有者的佔有行為的人，除非被剝奪或法律允許（非法（對財產的非法干擾））進行干涉。

下一部分不會賦予所有者從實際所有者那裡收回財產的例外，因為該物品不是 。

我個人建議要求家人將吉他交給您-而不是您進去拿吉他。

只需告訴他們您已將其借給他，但您需要將其退還。如果您描述了它，我相信他們會很樂意得到並交給您。或有違法行為；

聽起來，好像法律不是您需要擔心的部分-但是這個人傾向於積極進取。