First of all, nothing is totally secure. Computers are extremely complex, and software is extremely complex. The chance of no unknown security holes are basically nil.
Second, a password only protects the running operating system. Unless the disk is encrypted, it's trivial (<3 minutes) to remove the disk from the machine, and read whatever files you want.
Third, configuration has a lot of impact. Do you have an encrypted disk, but unencrypted swap? In that case the passphrase for encryption is probably stored somewhere in swap - especially if you have suspended the system to disk once.
Or do you have Firewire exposed on the computer? Firewire by default allows memory access, which means that an attacker may simply sift trough memory for your encryption keys...
Linux is no more secure than Windows to an attacker that has physical access to the machine - one could even argue that as Windows has TPM support enabled by default if you use Bitlocker, but Linux requires manual configuration of this - that a default Windows installation is safer against a local attacker.
When it comes to remote access (e.g. over a network), Linux distributions tend to fare rather well - simply because they don't run a lot of network services by default.
Define your threats, and try to mitigate them. If you worry about random thiefs getting access to your data, drive encryption and always powering laptop off before moving it or leaving it unattended will probably be enough. Against targeted espionage, that is probably not enough, as an attacker may for instance add logging devices while the laptop is unattended.
So in short - no, don't assume you're safe. Clearly define your threats, and the expected resources of the threat. A random thief will probably spend five minutes trying passwords, and move on and sell the hardware. CIA will be more sophisticated.