7-Zip doesn't ask me for a password for a ZIP file I encrypted while double-clicking it

The ZIP format doesn't allow for encrypting file lists. This means that file lists are viewable by anyone.

Only the contents of the files is encrypted, which means that no one can read the file without your password.

Due to this, 7-Zip only asks for your password before unzipping.

If you need a format that encrypts the file list, use 7Z and make sure you check "Encrypt File Names".

For the more technical minded, the ZIP specification doesn't allow for encryption of the Central Directory.

You will be prompted for a password when you try to extract the files. To simply view the contents of the archive does not require a password.

If you would like to obfuscate the contents of the archive, compress the directory into an archive, and then compress that archive with a password. Thus, you will have to extract the archive with a password, to pull out the archived (and obfuscated) contents.

A generalized solution that works for all zip programs (WINzip, 7-Zip, MacZip, etc.) is to simply zip it twice making sure the second time (or both) have password protected encryption.

See: Filenames display without entering the password when an encrypted Zip file is opened